[Iamit]

The manifestation is an idea postulated by Advaita that the many things appear, including us, but in reality are One. This idea only arises in the manifestation for there is nowhere else for anything to appear. Whether there is a reason for the manifestation is not addressed by Advaita but the idea that All is One is postulated as the only reality, so realization of this is regarded as Enlightenment.

Whether there is an earlier origin others may know more. It may be it was just made up by some priest in an attempt to obtain status and wealth. In any event it has endured and become the many faces of nonduality, most notable of which is the difference between the requirement that the nondual idea must be realized, and that it need not because no increase in connection to Oneness is achieved by realization as it is already Oneness not realizing.